In the production of heterogenous polymerization catalysts it is conventional to use for improving the polymerization activity of catalysts an inert carrier compound on which the actual catalytic component is formed. Among such carrier compounds there are both chemically inert carriers and carriers which have reactive chemical groups on their surface or within the carrying structure. Magnesium compounds, such as its oxides, oxy-halides and halides have proven to be important carrier compounds, the last-mentioned, especially MgCl.sub.2, having recently become the most important carrier compounds for isospecific catalysts.
The activity and stereospecificity of a catalyst greatly depends on the morphology of the carrier compound, i.e. on how the active groups of the catalyst component, deposited on the surface of the carrier, can settle in positions effective in terms of activity and stereospecificity. Although sufficiently fine comminution of the carrier improves the activity significantly, the shape of the carrier particles, determined by the structure of their crystal lattice, also has a great effect on the activity of the catalyst. Efforts have been made to control and improve this particle structure by both physical and chemical means. Magnesium halides have been prepared, for example, from organic Mg compounds such as Grignard reagents, which have been halogenated by various methods and in the presence of various auxiliary agents.
The carrier may also be ground by different methods, for example in a ball mill, to obtain the desired crystal structure. In such a case a magnesium halide, especially MgCl.sub., is ground in a ball mill together with an electron donor and possibly other compounds, even with the final catalyst components, for example for 50-120 hours at a temperature of 40.degree.-70.degree. C. As the result of the ball mill grinding there is typically obtained a finely-divided powder which has a large specific surface area and the particles of which have a deformed crystal lattice. When the powder is activated, for example by deposition by means of a transition metal compound and by reducing it by means of an organometal compound, a very active polymerization catalyst is obtained. However, a catalyst such as this, not having been treated further, has a disadvantage in its relatively low stereospecificity, and a polypropylene prepared by using the catalyst may, for example, contain an amorphous atactic fraction in a proportion up to 50%.
Thus, the carrier-grinding methods according to the state of the art are very slow and expensive and do not always lead to a suitable combination of catalyst activity and stereospecificity.